The schematic for this projec tis below. Be careful working with 220VAC because is quite dangerous so don't connect it to the main outle till you're not sure. Add the toggle swith btween the 220VAC input and the 5V converter. Insulate the converter module so it won't touch any low voltage part becasue that will damage the digital components. Once you have the case assembled, make the connections as below. Use thick wires for the power parts such as the 220V input and the relay + heater part. Add the pullup for the thermistor and a 10uF capacitor. Attatch the thermistor with good connection to the hot plate so it will measure the temperature. Supply all the digital part with 5V from the converter but check the output first wih a multimeter and make sure it's 5V, otherwise it might burn the Arduino and the rest.